Magento theme no longer responsive.

I recently upgraded a client site to Magento and I’ve encountered a few issues thus far. See my last post about the javascript type errors I had to troubleshoot a few weeks ago. The client site has been up and running Magento for a few weeks now and I noticed a responsive issue with the theme on iOS 11. Basically Magento is serving up the full site rather than a version suitable for smaller screen sizes. The fix for the javascript type error had to do with our custom theme missing a few javascript files (not pulling them in, rather). After taking a look at Wayback Machine, at a point in time when the responsive bits of the site did work, I noticed one small difference. The page source code is missing viewport meta tags.

<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1, user-scalable=no">